Welcome to the forum!
I would avoid DC and 495 altogether. Not sure about when you are planning on getting over to 15, but can you go locally up through Chantilly and Leesburg? Or get on I-66 west to I-81? This is a bit longer in length but will probably save you time IMO.
The rest of your route sounds great, and we have done different portions of that for different trips. [I-81 to I-84 to Newburgh then north on I-87]
If you are using EZ Pass don't forget that you can get a tag which can include your trailer, but not sure about the timing of getting it. Even with a one-axle trailer in theory you need to upgrade:
For our FC20 we recently got the "076" Tag Code EZ Pass. I think I just called them, because we had an EZ Pass account already, and they sent a new one right away.
PS -- As I look at the map, you could also stay on I-81 northbound to Binghamton NY then take I-88 to the Albany area, then pick up I-87. You would avoid lots of NYC traffic heading north on the NYS Thruway [I-87] on Friday. Not sure where the festival is, but I would tend to go this way if possible, I think, even if it may be a few more miles. Actually I-88 angles up toward Albany going in a ENE direction, so it may actually save a few miles IMO!
This could be a more scenic route also . . .